The present invention relates to a snowmobile, and more particularly to a device for protecting an electronic control unit from snow, water etc.
A recent snowmobile has been equipped with an electronic control system for controlling an engine. For example, in order to control the air-fuel ratio of a mixture for the engine, the fuel injection quantity is determined based on throttle valve opening degree .alpha. and engine speed N. A fuel injection pulse width for injecting fuel corresponding to the fuel injection quantity is calculated by an electronic control unit.
It is necessary to dispose the control unit in the snowmobile so as to protect the control unit from snow, rainwater and vibration of the engine. Furthermore, the control unit is provided to calculate the fuel injection pulse width based on output signals from sensors provided on the engine. Thus, it is desirable to locate the control unit near the engine, namely in an engine room.
Japanese Patent Publication 63-45359 discloses a snowmobile in which an engine room is formed in a front hood. An intake of air is formed on the front hood to take cold wind into the engine room to cool heat generating portions of the engine such as a muffler and an exhaust manifold, thereby preventing the fuel in a fuel pipe from vaporizing.
In such a snowmobile, snow and rainwater enter in the engine room together with the cold wind. If an electronic control unit is mounted in the engine room, the snow or the rainwater tend to stick to an outer wall of a case of the control unit.
Generally, the cover of the case and a body thereof are joined with each other through a packing for sealing therebetween. If the case is repeatedly heated by radiant heat and cooled by the atmosphere after stopping of the engine, the case breathes in accordance with the thermal expansion of the case. Thus, the water sticking on the outside of the case may enter the inside of the case. As a result, the drops of water adhere to a circuit board on which a CPU is mounted, causing malfunctioning of the control unit.
In order to solve these problems, the layout of the intake of air is changed to prevent snow and water from sticking to the case, or an additional cover is provided to cover the case to prevent the influence of the radiant heat.
However, the change of the air intake causes a reduction of the cooling efficiency of the engine room. The additional cover must be removed and attached at every maintenance and inspection operation of the control unit, which complicates the operation.
If the control unit is disposed adjacent the engine, the control unit is vibrated by the engine, causing erroneous operation of the control unit.